The extruder has overlapping functionality with the 'Path Duplicator', but lacks some of its features, and has additional ones of its own (it's a different kind of implementation, with different potential). Some significant strengths of the extruder are:

• it has 2d editing facilities (either a special 2d editing window, or circumference handles in a normal view) for controlling the shape of the outline, which can be a hollow or solid (including concave) pipe, a patch tube, or pipe with an outside square and inside complex outine).
• it has a variety of faclities for positioning path points with respect to each other (rmb on the path points).
• it can automatically punch holes in things that it goes thru, both for the inner and the outer circumference.

If you want to use the extruder 2d/outline editing features in combination with the features of the path duplicator, you can put a one-segment extruder into a path-duplicator's template group, and the shape will be extruded along the path.